An efficient, effective means for adhesively bonding low surface energy substrates such as polyethylene, polypropylene and polytetrafluoroethylene (e.g., TEFLON) has long been sought. The difficulties in adhesively bonding these materials are well known. See, for example, xe2x80x9cAdhesion Problems at Polymer Surfacesxe2x80x9d by D. M. Brewis that appeared in Progress in Rubber and Plastic Technology, volume 1, page 1 (1985).
The conventional approaches often use complex and costly substrate surface preparation techniques such as flame treatment, corona discharge, plasma treatment, oxidation by ozone or oxidizing acids and sputter etching. Alternatively, the substrate surface may be primed by coating it with a high surface energy material. However, to achieve adequate adhesion of the primer, it may be necessary to first use the surface preparation techniques described above. All of these techniques are well known, as reported in Treatis on Adhesion and Adhesives (J. D. Minford, editor, Marcel Dekker, 1991, New York, volume 7, pages 333-435). The known approaches are frequently customized for use with specific substrates. As a result, they may not be useful for bonding low surface energy substrates generally.
Moreover, the complexity and cost of the presently known approaches do not render them particularly suitable for use by the retail consumer (e.g., home repairs, do-it-yourselfers, etc.) or in low volume operations. One persistent problem is the repair of many inexpensive common household articles that are made of polyethylene, polypropylene or polystyrene such as trash baskets, laundry baskets and toys.
A series of patents issued to Zharov et al. (U.S. Pat. Nos. 5,539,070, 5,690,780 and 5,691,065) report a polymerizable acrylic bonding composition that comprises at least one acrylic monomer, an effective amount of certain organoborane amine complexes, and an effective amount of an acid for initiating polymerization of the acrylic monomer. The acrylic composition is especially useful as an acrylic adhesive for bonding low surface energy polymers.
Another series of patents issued to Pocius et al. (U.S. Pat. Nos. 5,616,796, 5,684,102 and 5,795,657) report polymerizable acrylic bonding compositions that comprise acrylic monomer, organoborane polyamine complex and a material reactive with amine. Polymerizable acrylic monomer compositions useful as adhesives for bonding low surface energy polymers can be prepared. The polyamine is the reaction product of a diprimary amine-terminated material, and a material having at least two groups reactive with a primary amine.
With increasingly demanding end-user requirements, bonding composition formulators are constantly being challenged to improve both application performance (e.g., worklife, rate of strength increase and cure time) and physical property performance (e.g., T-peel strength) of bonding compositions. It is very often times the case that a formulation change that enhances one property of a bonding composition deleteriously affects a second property of the bonding composition. Because of this, the formulator may have to accept less than a desirable balance between the competing properties. For this reason, adhesive formulators are constantly seeking new materials that provide a more favorable overall balance of properties in bonding compositions.
In many industrial and consumer applications for bonding compositions a long worklife is highly desirable feature. Worklife refers to the maximum time period available for bringing the bonding composition into contact with the substrate(s) to be bonded (i.e., closing the bond) after the initiation of the cure of the bonding composition. If the substrates are brought into contact with the bonding composition after the worklife has expired, the ultimate strength of the bond formed between the substrates may be compromised.
Several techniques have been reported for increasing the worklife of bonding compositions. In one known technique, worklife is increased by slowing the cure rate of the bonding composition, for example, by reducing the amount of polymerization initiator in the bonding composition and/or the chemical reactivity of the initiator. This technique, however, may typically lengthen the overall cure time and may slow the rate of strength increase of the bonding composition. The addition of certain polymerizable monomers to bonding compositions has also been reported to increase worklife. U.S. Pat. No. 5,859,160 (Righettini et al.) reports a free radical curable composition, useful as a two part adhesive, that includes a free radical curable compound and a vinyl aromatic compound that is chemically different than the free radical curable compound. The vinyl aromatic compound is present in an amount that is reportedly sufficient to decelerate the cure rate of the free radical composition without adversely effecting completion of cure and the properties of the curable composition after it has cured. In general, the amount of vinyl aromatic compound is less than 5 weight percent, preferably less than 2 weight percent, based on the total weight of the part of the composition that includes the free radical curable component. Although the above reported techniques may be used to increase the worklife of bonding compositions, other properties of the bonding composition such as rate of strength increase, cure time and T-peel strength may be sacrificed as a result of the increased worklife.
In addition to the foregoing, when formulating two-part bonding compositions it is very often desirable to formulate the parts such that they can be mixed with one another in a convenient mix ratio, for example, 1:1, 1:4, 1:10, and the like. To this end, materials are desired that may be added to one or more of the parts of the bonding composition to modify the mix ratio, wherein the addition of the materials does not deleteriously affect the performance characteristics and storage stability of the resulting bonding composition.
In one embodiment, the present invention provides polymerization initiator systems that are particularly useful in providing two-part curable bonding compositions, particularly those that cure (i.e., polymerize) to acrylic adhesives, more particularly those that cure to acrylic adhesives capable of bonding low surface energy substrates. The polymerization initiator systems of the present invention may be conveniently used to formulate two-part bonding compositions having a convenient whole number mix ratio. In addition, the polymerization initiator systems of the present invention enable the formulation of bonding compositions having a long worklife without substantially affecting other important properties such as rate of strength increase, cure time and T-peel strength. In preferred embodiments, the worklife of the bonding composition is increased and the T-peel strength of the cured bonding composition is also increased. Broadly, the polymerization initiator systems include an organoborane and at least one vinyl aromatic compound according to general formula (1) or general formula (2): 
In formula (1), n represents an integer having a value of 1 or greater, preferably 2 or greater. In formula (1) and formula (2), Ar represents a substituted aryl group. Examples of Ar include a substituted benzene group or a substituted napthalene group. Most preferably, Ar is a substituted benzene group.
In formula (1) and (2), subscript x, which represents an integer having a value of 1 or greater, represents the number of unsaturated groups bonded to each Ar group in the vinyl aromatic compound.
In formulas (1) and (2), R31, R32 and R33 are independently selected from the group consisting of hydrogen, alkyl, aryl and halogen. Preferably, R31 is selected from the group consisting of hydrogen and methyl and R32 and R33 are hydrogen.
In formulas (1) and (2), R34 represents a non-hydrogen substituent bonded to the aryl group (Ar). Subscript y is an integer having a value of 0 or greater which represents the number of individual substituents bonded to the aryl group Ar. When y is equal to 1 or greater, each substituent R34 may be independently selected from the group consisting of alkyl, alkoxy, alkanoyl, alkanoyloxy, aryloxy, aroyl, aroyloxy and halogen. Preferably, y is equal to 0 in formula (1).
In formula (1), X represents either a divalent organic linking group or a covalent bond. In a preferred embodiment, X is a divalent organic linking group comprising a urethane or a urea functional group.
In formula (1), R30 represents an organic group, preferably an oligomeric or polymeric organic group. The molecular weight of the (R30xe2x88x92Xn) group is about 100 or greater, more preferably about 200 or greater, and most preferably about 500 or greater. Representative examples of polymeric organic groups include hydrocarbon polymers (e.g., polyethylene, polystyrene, polypropylene, and polymethylpentene), carbon chain polymers (e.g., polyvinyl chloride, polyvinylidene chloride, and polyacrylonitrile), heterochain polymers (e.g., polyethers, polyamides, polyesters, polyurethanes, polysulfides, polysulfone, and polyimide).
The organoborane may be represented by the following general formula: 
where R1 is an alkyl group having 1 to about 10 carbon atoms. R2 and R3 may be the same or different and are independently selected from alkyl groups having 1 to about 10 carbon atoms and phenyl-containing groups. Preferred organoborane initiators are complexed with a complexing agent and may be represented by the following general formula: 
wherein R1, R2 and R3 are as described above, Cx is a complexing agent and v represent the ratio of complexing agent to boron atoms. Useful complexing agents (Cx) include, for example, amines, amidines, hydroxides and/or alkoxides.
The present invention is not limited, however, to bonding compositions prepared using the polymerization initiator systems of the present invention. Rather, the present invention broadly provides bonding compositions comprising an organoborane initiator, a polymerizable monomer and a vinyl aromatic compound, regardless of whether the vinyl aromatic compound is combined with the organoborane, the polymerizable monomer or both to form the bonding composition. Accordingly, another embodiment of the present invention provides bonding compositions comprising an organoborane, at least one polymerizable monomer, and at least one vinyl aromatic compound according to general formula (1) or (2) or a mixture thereof. The bonding compositions of the present invention may be used to bond a wide variety of substrates, but provide exceptionally good adhesion to low surface energy plastic substrates (e.g., polyethylene, polypropylene, polytetrafluoroethylene, etc.). Suitable polymerizable monomers include (meth)acrylates, for example, (meth)acrylic esters of monohydric alcohols and (meth)acrylic acid esters of polyhydric alcohols, acid amides, and mixtures thereof.
The bonding compositions of the present invention are typically and preferably provided in a two-part form wherein the initiator component is kept separate from the polymerizable monomer component. The two-parts are combined prior to application of the bonding composition to the substrate. Accordingly, in another embodiment, the present invention provides two-part curable bonding compositions comprising (a) a first part comprising an organoborane and (b) a second part comprising a polymerizable monomer. At least one of the first part or the second part further includes a vinyl aromatic compound according to general formula (1) or general formula (2). In a preferred embodiment, the vinyl aromatic compound is mixed with the organoborane to provide a polymerization initiator system of the present invention.
As used herein, the following terms have the following meanings.
The term xe2x80x9clow surface energy substratesxe2x80x9d are those substrates that have a surface energy of less than 45 mJ/m2, more typically less than 40 mJ/m2 or less than 35 mJ/ m2. Representative examples of low surface energy substrates include polyethylene, polypropylene and polytetrafluoroethylene.
The terms xe2x80x9cmonovalent organic groupxe2x80x9d and xe2x80x9cmultivalent organic groupxe2x80x9d mean an organic moiety. Monovalent organic groups have one available valency and multivalent organic groups have more than one available valency.
The term xe2x80x9corganic groupxe2x80x9d can be an aliphatic group or a cyclic group. In the context of the present invention, the term xe2x80x9caliphatic groupxe2x80x9d means a saturated or unsaturated, linear or branched, hydrocarbon. This term is used to encompass, for example, alkyl, alkylene, alkenyl, alkenylene, alkynyl and alkynylene groups.
The term xe2x80x9calkylxe2x80x9d means a monovalent, saturated, linear or branched, hydrocarbon group (e.g., a methyl, ethyl, isopropyl, t-butyl, heptyl, dodecyl, octadecyl, amyl, or 2-ethylhexyl group, and the like). The term xe2x80x9calkylenexe2x80x9d means a multivalent, saturated, linear or branched hydrocarbon group.
The term xe2x80x9calkenylxe2x80x9d means a monovalent, linear or branched, hydrocarbon group with one or more carbon-carbon double bonds (e.g., a vinyl group).
The term xe2x80x9calkenylenexe2x80x9d means a multivalent, linear or branched, hydrocarbon group with one or more carbon-carbon double bonds.
The term xe2x80x9calkynylxe2x80x9d means a monovalent, linear or branched, hydrocarbon group with one or more carbon-carbon triple bonds.
The term xe2x80x9calkynylenexe2x80x9d means a multivalent, linear or branched, hydrocarbon group with one or more carbon-carbon triple bonds.
The term xe2x80x9ccyclic groupxe2x80x9d means a closed ring hydrocarbon group that is classified as an alicyclic group, aromatic group, or heterocyclic group.
The term xe2x80x9calicyclic groupxe2x80x9d means a cyclic hydrocarbon group having properties resembling those of aliphatic groups.
The term xe2x80x9caromatic groupxe2x80x9d or xe2x80x9caryl -groupxe2x80x9d means a mononuclear aromatic hydrocarbon group or polynuclear aromatic hydrocarbon group.
The terms xe2x80x9corganic groupxe2x80x9d or xe2x80x9corganic linking groupxe2x80x9d may include, in addition to carbon and hydrogen, atoms of oxygen, nitrogen or sulfur which may be present, for example in the form of organic functional groups such as ethers, esters, amides, amines, aldehydes, ketones, carboxylic acids or carbonyls.
The term xe2x80x9calkoxyxe2x80x9d means an alkyl group bonded to an oxygen atom (i.e. an alkyl ether).
The term xe2x80x9calkanoylxe2x80x9d means an alkyl group bonded to a carbonyl group (i.e. an alkyl ketone).
The term xe2x80x9calkanoyloxyxe2x80x9d means an alkyl group bonded to a carbonyl group which is itself bonded to an oxygen atom, (i.e. an alkyl ester).
The term xe2x80x9caryloxyxe2x80x9d means an aryl group bonded to an oxygen atom (i.e. an aryl ether).
The term xe2x80x9caroylxe2x80x9d means an aryl group bonded to a carbonyl group (i.e. an aryl ketone).
The term xe2x80x9caroyloxyxe2x80x9d means an aryl group bonded to a carbonyl group which is itself bonded to an oxygen atom (i.e. an alkyl ester).
The present invention provides polymerization initiator systems that are particularly useful in providing two-part curable bonding compositions, especially those that cure (i.e., polymerize) to acrylic adhesives.
In one aspect of the invention, the polymerization initiator systems include an organoborane and at least one vinyl aromatic compound. The vinyl aromatic compound is advantageously both a carrier (extender) for the organoborane initiator and reactive with other components (e.g., polymerizable monomers) of the bonding composition. If the organoborane initiator is complexed, for example with an amine, a decomplexer which is preferably kept separate from the organoborane initiator until cure of the bonding composition is also necessary.
The polymerization initiator systems can be directly combined with polymerizable monomers for a two-part bonding composition in a convenient, commercially useful, whole number mix ratio of 1:10 or less. Moreover, and quite advantageously, the vinyl aromatic compound is reactive with the polymerizable monomers and can copolymerize therewith. Thus, in addition to providing a carrier or extender for the organoborane, the vinyl aromatic compound becomes incorporated into the polymerized bonding composition.
The individual components of the initiator systems and bonding compositions of the present invention are described below in detail below.
The organoborane initiates free-radical copolymerization of the polymerizable monomer and vinyl aromatic compound to form a polymer that can be useful as an bonding composition, for example an acrylic adhesive. The organoborane initiator may be represented by the following general formula: 
where R1 is an alkyl group having 1 to about 10 carbon atoms. R2 and R3 may be the same or different and are independently selected from alkyl groups having 1 to about 10 carbon atoms and phenyl-containing groups. Preferably, R1, R2 and R3 are independently selected from alkyl groups having 1 to about 5 carbon atoms. Accordingly, R1, R2 and R3 may all be different, or more than one of R1, R2 and R3 may be the same. Together, R1, R2 and R3, along with the boron atom (B) to which they are attached, form the initiator. Specific organoborane initiators include, for example, trimethylborane, triethylborane, tri-n-propylborane, triisopropylborane, tri-n-butylborane, triisobutylborane, and tri-sec-butylborane.
Preferred organoborane initiators are complexed with a complexing agent and may be represented by the following general formula: 
wherein R1, R2 and R3 are as described above and wherein Cx is a complexing agent.
Useful complexing agents (Cx) include, for example, amines, amidines, hydroxides and/or alkoxides. The ratio of complexing agent (Cx) to boron atoms in the complex is represented by xe2x80x9cvxe2x80x9d and is preferably selected so as to provide an effective ratio of the complexing agent and boron atoms. The complexing agent to boron atom ratio in the complex is preferably about 1:1. A complexing agent to boron atom ratio of less than 1:1 could leave free organoborane, a material that tends to be pyrophoric.
Amine complexing agents (Cx) may be provided by a wide variety of materials having at least one amine group, including blends of different amines. Amine complexing agents may also be polyamines (i.e., materials having two or more amine groups such as two to four amine groups).
In one embodiment the amine complexing agent may be a primary or secondary monoamine, such as those represented by the structure: 
wherein R4 and R5 are independently selected from the group consisting of hydrogen, alkyl groups having 1 to 10 carbon atoms, alkylaryl groups in which the amine group is not directly attached to the aryl structure, and polyoxyalkylene groups. Particular examples of these amines include ammonia, ethylamine, butylamine, hexylamine, octylamine, benzylamine, and polyoxyalkylene monoamines (e.g., JEFFAMINES from Huntsman Chemical Company, such as M715 and M2005).
In another embodiment, the amine may be a polyamine such as those described by the structure:
R5HNxe2x80x94R6xe2x80x94NHR5
wherein R5 is as defined above and wherein R6 is a divalent, organic radical comprised of an alkyl, aryl or alkylaryl group. Preferred among these materials are alkane diamines which may be branched or linear, and having the general structure 
in which x is a whole number greater than or equal to 1, more preferably about 2 to 12, and R7 is hydrogen or an alkyl group. Particularly preferred examples of alkane diamines include 1,2-ethanediamine, 1,3-propanediamine, 1,5-pentanediamine, 1,6-hexanediamine, 1,12-dodecanediamine, 2-methyl-1,5-pentane diamine, 3-methyl-1,5-pentane diamine, and isomers of these materials. While alkane diamines are preferred, other alkyl polyamines may be used such as triethylene tetraamine and diethylene triamine.
Useful polyamines may also be provided by a polyoxyalkylenepolyamine. Polyoxyalkylenepolyamines suitable in making complexes for the invention may be selected from the following structures:
H2NR8(R9O)wxe2x80x94(R10O)xxe2x80x94(R9O)yxe2x80x94R8NH2
(i.e., polyoxyalkylene diamines); or
[H2NR8xe2x80x94(R9O)w]zxe2x80x94R11.
R8, R9 and R10 are alkylene groups having 1 to 10 carbon atoms and may be the same or may be different. Preferably, R8 is an alkylene group having 2 to 4 carbon atoms such as ethylene, n-propylene, iso-propylene, n-butylene or iso-butylene. Preferably, R9 and R10 are alkylene groups having 2 or 3 carbon atoms such as ethylene, n-propylene or iso-propylene. R11 is the residue of a polyol used to prepare the polyoxyalkylenepolyamine (i.e., the organic structure that remains if the hydroxyl groups are removed). R11 may be branched or linear, and substituted or unsubstituted (although substituents should not interfere with oxyalkylation reactions).
The value of w is xe2x89xa71, more preferably about 1 to 150, and most preferably about 1 to 20. Structures in which w is 2, 3 or 4 are useful too. The value of x and y are both xe2x89xa70. The value of z is  greater than 2, more preferably 3 or 4 (so as to provide, respectively, polyoxyalkylene triamines and tetraamines). It is preferred that the values of w, x, y and z be chosen such that the resulting complex is a liquid at room temperature (xe2x80x9croom temperaturexe2x80x9d refers to, herein, a temperature of about 20 to 22xc2x0 C.) as this simplifies handling and mixing thereof. Usually, the polyoxyalkylenepolyamine is itself a liquid. For the polyoxyalkylenepolyamine, molecular weights of less than about 5,000 may be used, although molecular weights of about 1,000 or less are more preferred, and molecular weights of about 140 to 1,000 are most preferred.
Examples of particularly preferred polyoxyalkylenepolyamines include polyethyleneoxidediamine, polypropyleneoxidediamine, polypropyleneoxidetriamine, diethyleneglycoldipropylamine, triethyleneglycoldipropylamine, polytetramethyleneoxidediamine, poly(ethyleneoxide-co-propyleneoxide)diamine, and poly(ethyleneoxide-co-propyleneoxide)triamine.
Examples of suitable commercially available polyoxyalkylenepolyamines include various JEFFAMINES from Huntsman Chemical Company such as the D, ED, and EDR series diamines (e.g., D-400, D-2000, D-5000, ED-600, ED-900, ED-2001, and EDR-148), and the T series triamines (e.g., T-403), as well as DCA-221 from Dixie Chemical Company.
As reported in U.S. Pat. No. 5,616,796 (Pocius et al.), the disclosure of which is incorporated herein by reference, the polyamine may also comprise the condensation reaction product of diprimary amine-terminated material (i.e., the two terminal groups are primary amine) and one or more materials containing at least two groups reactive with primary amine.
Hydroxide and/or alkoxide complexing agents (Cx) are reported in copending application having U.S. Ser. No. 09/433,476 (Moren), filed Nov. 4, 1999 pending the disclosure of which is incorporated herein by reference. Preferred hydroxide and/or alkoxide complexing agents may be represented by the formula:
((xe2x88x92)Oxe2x80x94R15)nM(m+)
wherein:
R15 is independently selected from hydrogen or an organic group (e.g., alkyl or alkylene group);
M(m+) represents a countercation (e.g., sodium, potassium, tetraalkylammonium, or combinations thereof);
n is an integer greater than zero; and
m is an integer greater than zero.
Amidine complexing agents may be represented by the formula: 
wherein:
R16 is hydrogen or an organic group, preferably hydrogen or an alkyl or alkylene group;
R17 and R18 are independently a monovalent organic group or part of a cyclic structure; and
w, x, and y comprise integers, preferably w being 1 and x being about 3 or less.
Particularly preferred amidine complexing agents comprise those selected from the group consisting of N,N,Nxe2x80x2,Nxe2x80x2-tetramethylguanidine; 1,8-diazabicyclo[5.4.0]undec-7-ene; 1,5-diazabicyclo[4.3.0]non-5-ene; 2-methylimidazole; 2-methylimidazoline; and 4-(N,N-dimethylamino)-pyridine.
An organoborane complex may be readily prepared using known techniques. Typically, the complexing agent is combined with the organoborane in an inert atmosphere (e.g., a glovebox flushed with nitrogen to an environment having less than 100 ppm oxygen) with slow stirring. The organoborane can be added from a pressure equalizing dropping funnel to a flask into which the coupling agent has been previously weighed. An exotherm is often observed and cooling of the mixture is, therefore, recommended. Addition of the organoborane may be moderated to control the exotherm. If the ingredients have a high vapor pressure, it is desirable to keep the reaction temperature below about 70xc2x0 to 80xc2x0 C. Once the materials have been well mixed the complex is permitted to cool to room temperature. No special storage conditions are required although it is preferred that the complex be kept in a capped vessel in a cool, dark location. A crystalline mass of the complex can be heated (e.g., to about 55xc2x0 C.) with an oil bath and outside of the nitrogen environment to liquify the complex and facilitate its transfer to the storage vial, which can be flushed with nitrogen.
The organoborane is employed in an effective amount, which is an amount large enough to permit acrylic monomer polymerization to readily occur to obtain an acrylic polymer of high enough molecular weight for the desired end use. If the amount of organoborane is too low, then the polymerization may be incomplete or, in the case of adhesives, the resulting composition may have poor adhesion. On the other hand, if the amount of organoborane is too high, then the polymerization may proceed too rapidly to allow for effective mixing and use of the resulting composition.
Large amounts of organoborane could potentially weaken the bond formed by a bonding composition of the present invention. The useful rate of polymerization will depend in part on the method of applying the composition to a substrate. Thus, a faster rate of polymerization may be accommodated by using a high speed automated industrial adhesive applicator rather than by applying the composition with a hand applicator or by manually mixing the composition.
Within these parameters, an effective amount of the organoborane is an amount that preferably provides about 0.003 to 1.5%-wt. boron, more preferably about 0.008 to 0.5%-wt. boron, most preferably about 0.01 to 0.3%-wt. boron. The %-wt. of boron in a composition is based on the total weight of the composition, less fillers, non-reactive diluents, and other non-reactive materials. Thus, the polymerizable monomers, the vinyl aromatic compound, and organic thickener, (e.g., poly(methyl methacrylate) or core-shell polymer), if present, are included, but ingredients lacking abstractable hydrogen atoms or unsaturation are not. The %-wt. of boron in the composition may be calculated by the following equation:                                           (                          weight              ⁢                              xe2x80x83                            ⁢              of              ⁢                              xe2x80x83                            ⁢              organoborane                                                                                      in              ⁢                              xe2x80x83                            ⁢              the              ⁢                              xe2x80x83                            ⁢              composition                        )                                xc3x97                                        (                          %              ⁢                              -                            ⁢                              wt                .                                  xe2x80x83                                ⁢                of                            ⁢                              xe2x80x83                            ⁢              boron                                                                                      in              ⁢                              xe2x80x83                            ⁢              the              ⁢                              xe2x80x83                            ⁢              organoborane                        )                                                                        (                          Total              ⁢                              xe2x80x83                            ⁢              weight              ⁢                              xe2x80x83                            ⁢              of              ⁢                              xe2x80x83                            ⁢              the              ⁢                              xe2x80x83                            ⁢              composition                        ⁢                          xe2x80x83                                                                                      less              ⁢                              xe2x80x83                            ⁢              non              ⁢                              -                            ⁢              reactive              ⁢                              xe2x80x83                            ⁢              components                        )                                ⁢          xe2x80x83      
Quite advantageously, the organoborane is carried by (e.g., dissolved in or diluted by) a vinyl aromatic compound or a blend of two or more different vinyl aromatic compounds. The vinyl aromatic compound should not be reactive toward the complexing agent and functions as an extender for the organoborane.
The vinyl aromatic compound should be soluble in acrylic monomers included in the bonding composition. By xe2x80x9csolublexe2x80x9d it is meant that no evidence of gross phase separation at room temperature is visible to the unaided eye. Similarly, the organoborane should be soluble in the vinyl aromatic compound, although slightly warming a mixture of the organoborane and the vinyl aromatic compound may be helpful in forming a solution of the two at room temperature. Preferably the vinyl aromatic compound is a liquid at or near room temperature (i.e., within about 10xc2x0 C. of 20-22xc2x0 C.) or forms a liquid solution with the organoborane at or near room temperature. Higher viscosity vinyl aromatic compounds may also be useful. Compounds having a Brookfield viscosity of up to about 1,000,000 cp at 22xc2x0 C. may be successfully employed, although materials with a viscosity of about 100,000 cp or less are more preferred.
The utility of vinyl aromatic compounds as carriers or extenders in the present invention is enhanced by employing materials that show little or no volatility at room temperature (no appreciable or readily measurable change in volume after 6 months storage at room temperature). Such materials generally have a boiling point in excess of about 160xc2x0 C., more preferably in excess of about 190xc2x0 C., and most preferably greater than about 210xc2x0 C.
The vinyl aromatic compounds impart excellent storage stability and an extended shelf-life to initiation systems and polymerizable compositions made therewith. That is, the initiator system and polymerizable compositions remain stable at room temperature for an extended period of time. Thus, special storage conditions such as refrigeration can be avoided without substantially sacrificing the storage life of the product.
Quite advantageously, substantial amounts (e.g., up to 50% by weight) of the organoborane may be dissolved in the vinyl aromatic compound, which facilitates the provision of two-part adhesives that can be combined in a commercially useful mix ratio. The vinyl aromatic compound also functions as a reactive extender because the ethylenic unsaturation enables this material to free-radically copolymerize with acrylic monomers. Advantageously, this yields a fully (i.e., 100%) reactive system, sometimes referred to herein as a 100% solids system. Desirably, this can reduce the level of low molecular weight migratory components in the polymerizable composition which, in the case of an adhesive, could bloom to the surface of a bonded interface and reduce the strength of the adhesive bond.
When complexed organoborane initiators are used as the organoborane initiator in a bonding compositions of the present invention, the bonding compositions further comprise a decomplexer. The term xe2x80x9cdecomplexerxe2x80x9d as used herein refers to a compound capable of liberating the initiator (e.g., organoborane) from its complexing agent, thereby enabling initiation of the polymerizable monomer of the bonding composition. Decomplexers may also be referred to as xe2x80x9cactivatorsxe2x80x9d or xe2x80x9cliberatorsxe2x80x9d and these terms may be used synonymously herein.
When the organoborane is complexed with an amine, a suitable decomplexer is an amine reactive compound. The amine reactive compound liberates organoborane by reacting with the amine, thereby removing the organoborane from chemical attachment with the amine. A wide variety of materials may be used to provide the amine reactive compound including combinations of different materials. Desirable amine reactive compounds are those materials that can readily form reaction products with amines at or below room temperature (about 20xc2x0 to 22xc2x0 C.) so as to provide a composition such as an adhesive that can be easily used and cured under ambient conditions. General classes of useful amine reactive compounds include acids, anhydrides and aldehydes. Isocyanate, acid chloride, sulfonyl chloride, and the like such as isophorone diisocyanate, toluene diisocyanate and methacryloyl chloride may also be used.
Any acid that can liberate the organoborane by salting the amine group may be employed. Useful acids include Lewis acids (e.g., SnCl4, TiCl4 and the like) and Bronsted acids (e.g., carboxylic acids, HCl, H2SO4, H3PO4, phosphonic acid, phosphinic acid, silicic acid, and the like). Useful carboxylic acids include those having the general formula R20xe2x80x94COOH, where R20 is hydrogen, an alkyl group of 1 to 8 and preferably 1 to 4 carbon atoms, or an aryl group of 6 to 10, preferably 6 to 8 carbon atoms. The alkyl groups may comprise a straight chain or they may be branched. They may be saturated or unsaturated. The aryl groups may contain substituents such as alkyl, alkoxy or halogen moieties. Illustrative acids of this type include acrylic acid, methacrylic acid, acetic acid, benzoic acid, and p-methoxybenzoic acid.
If it is desirable to provide a polymerizable composition that has less odor, an alkenyl group having a larger number of carbon atoms is recommended. In this event, R20 may be a straight or branched chain, saturated or unsaturated alkenyl group of at least 9 carbon atoms, more preferably at least about 11 carbon atoms, and most preferably at least about 15 carbon atoms.
Other carboxylic acids useful as the amine reactive compound include dicarboxylic acids and carboxylic acid esters. Such compounds may be represented by the following general structure: 
R21 is hydrogen, a monovalent organic group (preferably having about 18 atoms or less, more preferably about 8 atoms or less), or a multivalent organic group (preferably having about 30 atoms or less, more preferably about 10 atoms or less). R22 is multi-valent organic group (preferably having about 8 atoms or less, more preferably about 4 atoms or less). R23 is hydrogen or a monovalent organic group (preferably having about 18 atoms or less, more preferably about 8 atoms or less). The integral value of xe2x80x9cmxe2x80x9d is 0, 1 or 2, and the integral value of xe2x80x9cnxe2x80x9d is greater than or equal to one, preferably 1 to 4, more preferably 1 or 2.
More preferably m is 0 so as to yield carboxylic acids represented by the following general structure: 
wherein R21, R22, and n are as previously defined.
The xe2x80x9corganic groupsxe2x80x9d referred to in conjunction with R21, R22 and R23 may be an aliphatic group (which may be saturated or unsaturated, and linear or branched), a cycloaliphatic group, an aromatic group, or an oxygen-, nitrogen-, or sulfur- containing heterocyclic group. When R21 is hydrogen, m is zero, and n is one, the resulting compounds are dicarboxylic acids, useful examples of which include: oxalic acid; malonic acid; succinic acid; glutaric acid; adipic acid; maleic acid; fumaric acid; phthalic acid; isophthalic acid; and terephthalic acid. When, R21 is an aliphatic group, n is one, and m is zero, the resulting compounds are carboxylic acid esters, useful examples of which include: 1,2-ethylene bismaleate; 1,2-propylene bismaleate; 2,2xe2x80x2-diethyleneglycol bismaleate; 2,2xe2x80x2-dipropyleneglycol bismaleate; and trimethylolpropane trismaleate.
Also preferred as the amine reactive compound are materials having at least one anhydride group, such materials preferably having one of the following structures: 
R24 and R25 are organic radicals which independently may be aliphatic (including straight- and branched-chain arrangements that may be saturated or unsaturated), cycloaliphatic, or aromatic. Preferred aliphatic groups comprise 1 to 17 carbon atoms, more preferably 2 to 9 carbon atoms. Preferred aromatic groups include benzene which may be substituted with 1 to 4 carbon atom aliphatic groups.
R26 is a divalent organic radical that completes a cyclic structure with the anhydride group to form, for example, a 5- or 6-membered ring. R26 may be substituted with aliphatic, cycloaliphatic or aromatic groups, preferably aliphatic groups comprising 1 to 12, more preferably 1 to 4 carbon atoms. R26 may also contain heteroatoms such as oxygen or nitrogen provided that any heteroatom is not adjacent to the anhydride functionality. R26 may also be part of a cycloaliphatic or aromatic fused ring structure, either of which may be optionally substituted with aliphatic groups. The presence of a free-radically polymerizable group in the anhydride-functional amine reactive compound may permit the same to polymerize with the acrylic monomers.
Aldehydes useful as the amine-reactive compound have the formula:
xe2x80x83R27xe2x80x94(CHO)x
where R27 is a monovalent organic radical, such as is an alkyl group of 1 to 10 carbon atoms (preferably 1 to 4), or an aryl group having 6 to 10 carbon atoms (preferably 6 to 8), and x is 1 or 2 (preferably 1). In this formula, the alkyl groups may be straight or branch-chained, and may contain substituents such as halogen, hydroxy and alkoxy. The aryl groups may contain substituents such as halogen, hydroxy, alkoxy, alkyl and nitro. The preferred R27 group is aryl. Illustrative examples of compounds of this type include, benzaldehyde, o-, m- and p-nitrobenzaldehyde, 2,4-dichlorobenzaldehyde, p-tolylaldehyde and 3-methoxy-4 hydroxybenzaldehyde. Blocked aldehydes such as acetals may also be used in this invention.
The decomplexer is employed in an effective amount (i.e., an amount effective to promote polymerization by liberating the initiator from its complexing agent, but without materially adversely affecting desired properties of the ultimate polymerized composition). As recognizable to one of ordinary skill in the art, too much of the decomplexer may cause polymerization to proceed too quickly and, in the case of adhesives, the resulting materials may demonstrate inadequate adhesion to low energy surfaces. However, if too little decomplexer is used, the rate of polymerization may be too slow and the resulting polymers may not be of adequate molecular weight for certain applications. A reduced amount of decomplexer may be helpful in slowing the rate of polymerization if it is otherwise too fast. Thus, within these parameters, the decomplexer is typically provided in an amount such that the ratio of amine-, amidine-, hydroxide- or alkoxide-reactive groups in the decomplexer(s) to amine, amidine, hydroxide or alkoxide groups in the complexing agent(s) is in the range of 0.5:1.0 to 3.0:1.0. For better performance, preferably the ratio of amine-, amidine-, hydroxide- or alkoxide-reactive groups in the decomplexer(s) to amine, amidine, hydroxide or alkoxide groups in the complexing agent(s) is in the range of 0.5:1.0 to 1.0:1.0, preferably about 1.0:1.0.
A xe2x80x9cvinyl aromatic compoundxe2x80x9d refers to an organic compound according to general formula (1) or general formula (2) or a mixture thereof: 
In formula (1), n represents an integer having a value of 1 or greater, preferably 2 or greater. In formula (1) and formula (2), Ar represents a substituted aryl group, preferably having from 6-10 carbon atoms. Examples of Ar include a substituted benzene group having the formula C6H5xe2x88x92xxe2x88x92y for formula (1) or C6H6xe2x88x92xxe2x88x92y for formula (2) or a substituted napthalene group having the formula C10H7xe2x88x92xxe2x88x92y for formula (1) or C10H8xe2x88x92xxe2x88x92y for formula (2). Most preferably, Ar is a substituted benzene group.
In the vinyl aromatic compounds of formulas (1) and (2), the xe2x80x94CR31=CR32R33 group provides a site of unsaturation (i.e., a double bond) which is reactive with the polymerizable monomer of the bonding composition. That is, the vinyl aromatic compound copolymerizes with the polymerizable monomer and becomes chemically attached to the polymerizable monomer. In formula (1) and (2), subscript x, which represents an integer having a value of 1 or greater, represents the number of unsaturated moieties bonded to each Ar group in the vinyl aromatic compound. In a preferred embodiment of formula (1), x is 1.
In formulas (1) and (2), R31, R32 and R33 are independently selected from the group consisting of hydrogen, alkyl, aryl and halogen. Preferably, R31 is selected from the group consisting of hydrogen and methyl and R32 and R33 are hydrogen. To avoid gelling, it is generally preferred that vinyl aromatic compounds of formulas (1) and (2) having R31=H, are packaged separate from the organoborane (e.g., included only in part B) in two part bonding compositions of the present invention.
In formulas (1) and (2), R34 represents a non-hydrogen substituent bonded to the aryl group Ar. Subscript y is an integer having a value of 0 or greater which represents the number of individual substituents bonded to the aryl group Ar. When y is equal to 1 or greater, each substituent R34 may be independently selected from the group consisting of alkyl, alkoxy, alkanoyl, alkanoyloxy, aryloxy, aroyl, aroyloxy and halogen. Preferably, y is equal to 0 in formula (1).
In formula (1), X represents either a divalent organic linking group or a covalent bond. In a preferred embodiment, X is a divalent organic linking group comprising a urethane or a urea functional group. In a more preferred embodiment, X is: 
wherein R35 and R36 are divalent organic linking groups having from 1-10 carbon atoms. If present, R35 and R36 are bonded to the aryl group (Ar) of formula (1).
In formula (1), R30 represents an organic group, preferably an oligomeric or polymeric organic group. The molecular weight of R30xe2x80x94Xn is 100 or greater, more preferably 200 or greater, and most preferably 500 or greater. Representative examples of polymeric organic groups include hydrocarbon polymers (e.g., polyethylene, polystyrene, polypropylene, and polymethylpentene), carbon chain polymers (e.g., polyvinyl chloride, polyvinylidene chloride, and polyacrylonitrile), heterochain polymers (e.g., polyethers, polyamides, polyesters, polyurethanes, polysulfides, polysulfone, and polyimide). Suitable polymeric organic groups may be homopolymers or copolymers, for example, copolymers and terpolymers and may be alternating, random, block, or graft in structure. Preferred organic groups R30 include polyesters (e.g., polycaprolactone) having a molecular weight ranging from about 300-1000 (grams/mole) and polyethers having a molecular weight ranging from about 500-3000 (grams/mole).
Preferred monofunctional vinyl aromatic compounds of formula (1) are represented below in general formula (1A) wherein, with reference to formula (1), Ar is a benzene ring, y is 0, R31 is methyl, R32 and R33 are hydrogen, x is 1, and n is 1. The bonding structure to the benzene ring is shown generally and may be ortho, meta or para. 
Representative examples of monofunctional vinyl aromatic compounds of formula (1A) include: 
wherein m typically ranges from about 0 to 50; and n typically ranges from about 0 to 48.
In one embodiment, for example, m is equal to 6 and n is equal to 38.
Preferred difunctional vinyl aromatic compounds of formula (1) are represented below in general formula (1B) wherein, with reference to formula (1), Ar is a benzene ring, y is 0, R31 is methyl, R32 and R33 are hydrogen, x is 1, and n is 2. The bonding structure to the benzene rings is shown generally and may be independently on each ring ortho, meta or para. 
Representative examples of difunctional vinyl aromatic compounds of formula (1B) include: 
wherein m typically ranges from about 0 to 50; and n typically ranges from about 0 to 50; 
wherein n typically ranges from about 0 to 140; and R37 is methyl or hydrogen.
Preferred trifunctional vinyl aromatic compounds of formula (1) are represented below as general formula (1C) wherein, with reference to formula (1), Ar is a benzene ring, y is 0, R31 is methyl, R32 and R33 are hydrogen, x is 1, and n is 3. The bonding structure to the benzene rings is shown generally and may be independently on each ring ortho, meta or para. 
Representative examples of the trifunctional vinyl aromatic compounds of formula (1C) include: 
wherein (n+m) typically ranges from about 5 to 85; 
wherein (n+m) typically ranges from about 2 to 18.
Useful vinyl aromatic compounds of general formula (1) may be prepared, for example, by reacting 3-isopropenyl-xcex1,xcex1-dimethylbenzyl isocyanate (commerically available under the trade designation xe2x80x9cTMIxe2x80x9d from Cytec Industries, West Peterson, N.J.) with a mono- or multi-functional reactive hydrogen compound, preferably a mono- or multi-functional amine, alcohol or combination thereof. Particularly preferred mono- and multi-functional amines include the amine terminated polyethers commercially available under the trade designation xe2x80x9cJEFFAMINExe2x80x9d (from Huntsman Chemical Co., Houston, Tex.) for example xe2x80x9cJEFFAMINE ED600xe2x80x9d (a diamine terminated polyether having a reported molecular weight of 600) xe2x80x9cJEFFAMINE D400xe2x80x9d (a diamine terminated polyether having a reported molecular weight of 400), xe2x80x9cJEFFAMINE D2000xe2x80x9d (a diamine terminated polyether having a reported molecular weight of 2000), xe2x80x9cJEFFAMINE T3000xe2x80x9d (a triamine terminated polyether having a reported molecular weight of 3000), and xe2x80x9cJEFFAMINE M2005xe2x80x9d (a monoamine terminated polyether having a reported molecular weight of 2000). Suitable alcohol-containing compounds include, for example, polypropylene glycol, polycaprolactone triol, diethylene glycol.
When the vinyl aromatic compound is synthesized as the reaction product of an alcohol with an isocyanate, it may be desirable to use a catalyst to speed the reaction between the isocyanate and the alcohol. Suitable catalysts are well known in the art and include, for example, dibutyltin dilaurate (DBTDL) (commercially available from Aldrich Chemical Co., Milwaukee, Wis.). Additional details pertaining to isocyanate catalysis may be found in Polyurethanes: Chemistry and Technology, Saunders and Frisch, Interscience Publishers (New York, 1963 (Part I) and 1964 (Part II)).
Representative examples of vinyl aromatic compounds of formula (2) include: 
It may be desirable in some instances to add a free radical stabilizer to the vinyl aromatic compound of formula (1) or (2), particularly to vinyl aromatic compounds wherein R31 is hydrogen. A free radical stabilizer functions to prevent premature free radical polymerization of the vinyl aromatic compound. One such free radical stabilizer is 2,6-di-tert-butyl-4-methylphenol (commercially available from Aldrich Chemical Co., Milwaukee, Wis.). When a free radical stabilizer is used, it is typically added in an amount ranging from about 10 to 5000 ppm.
Bonding compositions of the present invention comprise a vinyl aromatic compound of general formula (1) or (2) or a mixture thereof in an effective amount in order to provide the desired balance of worklife, rate of strength increase and cured bonding composition physical properties. The vinyl aromatic compound is used in an effective amount that does not materially, adversely affect the ultimate properties of the polymerized composition (for example, adhesion), depending on the intended use. Generally, for the vinyl aromatic compounds of formula (1), this is an amount of about 1%-wt. or greater, preferably about 5%-wt. or greater, more preferably about 5-25%-wt., based on the total weight of the bonding composition. For the vinyl aromatic compounds of formula (2) wherein R31 is not hydrogen (e.g., a methyl group), this is an amount of about not more than about 1%-wt., more preferably not more than about 0.5%-wt. For the vinyl aromatic compounds of formula (2) wherein R31 is hydrogen, this is an amount of about 1%-wt. or greater, preferably about 3%-wt. or greater, and most preferably about 5 to 15%-wt. or greater. Preferred bonding compositions retain at least 90% or greater, more preferably 95% or greater, and most preferably 99% or greater overlap shear strength at 10 minutes of open time (see, Overlap Shear Strength Test Method).
Bonding compositions of the present invention include at least one polymerizable monomer. Broadly, the polymerizable monomer in a bonding composition of the present invention includes at least one ethylenically unsaturated monomer capable of free radical polymerization. Numerous compounds containing ethylenic unsaturation can be used in the bonding composition. Preferably, the composition includes at least one (meth)acrylic monomer, most preferably at least one methacrylic monomer. Particularly preferred are (meth)acrylic acid derivatives, such as those including esters and/or acid amides. Suitable are, for example, the (meth)acrylic esters of monohydric alcohols, particularly alkanols having from 1 to 12 carbon atoms, such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, isooctyl (meth)acrylate, isobomyl (meth)acrylate, isodecyl (meth)acrylate, ethylhexyl (meth)acrylate; the (meth)acrylic esters of monohydric alcohols further including heteroatoms, such as tetrahydrofurfuryl (meth)acrylate and 2-ethoxyethyl (meth)acrylate; the (meth)acrylic acid esters of polyhydric alcohols, such as ethylene glycol, diethylene glycol, polyethylene glycol, trimethylol propane, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, pentapropylene glycol and polypropylene glycol; ethoxylated or propoxylated diphenylolpropane and hydroxy-terminated polyurethanes. (Meth)acrylic acid esters of polyhydric alcohols are hereinafter referred to as oligomeric (meth)acrylates.
Basically suitable are also polymerizable monomers, such as vinyl acetate; vinyl halides, such as vinyl chloride, vinyl fluoride, and vinyl bromide. These compounds, however, are generally used only in subordinate amounts in the polymerizable compositions.
Further suitable polymerizable monomers are acid amides, such as acrylamide; N-methyl(meth)acrylamide; N,N-dimethyl(meth)acrylamide; N-ethyl(meth)acrylamide; N,N-diethyl(meth)acrylamide; N-isopropyl(meth)acrylamide; N-butyl(meth)acrylamide; N-t-butyl(meth)acrylamide; N,N-dibutyl(meth)acrylamide; N-phenyl(meth)acrylamide; N-((meth)acryloyl)morpholine; N-((meth)acryloyl)piperidine; N-(1,1-dimethyl-3-oxobutyl)(meth)acrylamide; N-1,1,3,3-tetramethylbutyl(meth)acrylamide; methylene-bis-(meth)acrylamide; tetramethylene-bis-(meth)acrylamide; trimethylhexamethylene-bis-(meth)acrylamide; tri(meth)acryloyldiethylenetriamine; and similar compounds. Preferred acid amides include N,N-dimethyl(meth)acrylamide; N,N-diethyl(meth)acrylamide; N-butyl(meth)acrylamide; N,N-dibutyl(meth)acrylamide; N-((meth)acryloyl)morpholine; and N-((meth)acryloyl)piperidine.
In general, the emphasis is on monomers with one or two olefinic double bonds in the molecule, preferably one olefinic double bond. The additional use of higher unsaturated components is not excluded, but it must be kept in mind that their presence may adversely affect worklife and/or physical performance.
A preferred blend of monomers comprises 10-90%-wt. M1, 25-70%-wt. M2, and 0-65%-wt. M3 based on the total weight of the monomer blend, wherein:
M1 is tetrahydrofurfuryl methacrylate;
M2 is one or more monomers selected from the group consisting of 2-ethoxyethyl methacrylate, isooctyl acrylate, 2-ethylhexyl (meth)acrylate, and isobomyl acrylate; and
M3 is one or more monomers selected from the group consisting of isobornyl methacrylate, and isodecyl methacrylate.
When vinyl aromatic compounds according to general formula (2) are employed in bonding compositions of the present invention, the polymerizable monomer blend preferably further comprises an oligomeric (meth)acrylate monomer prepared from a polyhydric alcohol selected from the group consisting of polyethylene glycol, polypropylene glycol, ethoxylated diphenylolpropane, propoxylated diphenylolpropane and hydroxy-terminated polyurethanes.
Bonding compositions of the present invention may further comprise optional additives. One particularly useful additive is a thickener, such as medium (i.e., about 40,000) molecular weight polybutyl methacrylate that may generally be incorporated in an amount of up to about 50%-wt., based on the total weight of the polymerizable monomer. Thickeners may be employed to increase the viscosity of the resulting bonding composition to a more easily applied viscous syrup-like consistency.
Another particularly useful additive is an elastomeric material. These materials can improve the fracture toughness of bonding compositions made therewith, which can be beneficial when, for example, bonding stiff, high yield strength materials (e.g., metal substrates that do not mechanically absorb energy as easily as other materials, such as flexible polymeric substrates). Such additives can generally be incorporated in an amount of up to about 50%-wt., based on the total weight of the bonding composition.
Core-shell polymers can also be added to modify spreading and flow properties of the bonding composition. These enhanced properties may be manifested by a reduced tendency for the bonding composition to leave an undesirable xe2x80x9cstringxe2x80x9d upon dispensing from a syringe-type applicator, or sag or slump after having been applied to a vertical surface. Accordingly, use of more than about 20%-wt., based on total weight of the bonding composition, of a core-shell polymer additive may be desirable for achieving improved sag-slump resistance. Core-shell polymers can also improve the fracture toughness of bonding compositions made therewith, which can be beneficial when, for example, bonding stiff, high yield strength materials (e.g., metal substrates that do not mechanically absorb energy as easily as other materials, such as flexible polymeric substrates).
Reactive diluents may also be added to bonding compositions of the present invention. Suitable reactive diluents include 1,4-dioxo-2-butene functional compounds as reported in U.S. Ser. No. 09/272,152 U.S. Pat. No. 6,252,023 Jun. 26, 2001 (Moren) and aziridine functional compounds as reported in U.S. Pat. No. 5,935,711 (Pocius et al.), the disclosures of which are incorporated herein by reference.
Small amounts of inhibitors, such as hydroquinone monomethyl ether may be used in the polymerizable compositions, for example, to prevent or reduce degradation of the polymerizable monomers during storage. Inhibitors may be added in an amount that does not materially affect the rate of polymerization or the ultimate properties of polymers made therewith. Accordingly, inhibitors are generally useful in amounts of about 100-10,000 ppm based on the total weight of the polymerizable monomers in the polymerizable composition.
Other possible additives include non-reactive colorants, fillers (e.g., carbon black, hollow glass/ceramic beads, silica, titanium dioxide, solid glass/ceramic spheres, electrically and/or thermally conductive particulate, antistatic compounds, and chalk), and the like. The various optional additives are employed in any amount, but generally amounts that do not significantly adversely affect the polymerization process or the desired properties of polymers made therewith.
Bonding compositions of the invention are especially useful for adhesively bonding low surface energy plastic or polymeric substrates that historically have been very difficult to bond without using complicated surface preparation techniques, for example, priming. By low surface energy substrates is meant materials that have a surface energy of less than 45 mJ/m2, more typically less than 40 mJ/m2 or less than 35 mJ/m2. Included among such materials are polyethylene, polypropylene, acrylonitrile-butadiene-styrene, and fluorinated polymers such as polytetrafluoroethylene (TEFLON) which has a surface energy of less than 20 mJ/m2. (The expression xe2x80x9csurface energyxe2x80x9d is often used synonymously with xe2x80x9ccritical wetting tensionxe2x80x9d by others.) Other polymers of somewhat higher surface energy that may be usefully bonded with the compositions of the invention include polycarbonate, polymethylmethacrylate, and polyvinylchloride.
The bonding compositions of the invention can be easily provided as two-part formulations. The acrylic monomers are blended as would normally be done when working with such materials. The bonding compositions of the present invention are preferably provided in two-part formulation with the parts being mixed prior to application of the bonding composition to a substrate. In this way, the polymerizable monomers may be separated from the organoborane initiator until cure (i.e., polymerization) of the bonding composition is desired. Accordingly, the first part or xe2x80x9cPart Axe2x80x9d of the two-part bonding composition comprises an organoborane initiator (preferably a complexed organoborane initiator) and may further comprise optionally additives, for example, a reactive diluent or plasticizer. The second part or xe2x80x9cPart Bxe2x80x9d of the two-part bonding composition comprises at least one polymerizable monomer, and further comprises a decomplexer in the case where the organoborane initiator in Part A is complexed (e.g., an organoborane amine complex). Part B part may further comprise optional additives, for example, microspheres or a core-shell polymer. In bonding compositions of the present invention, a vinyl aromatic compound is included in Part A, Part B or both Part A and Part B.
For a two-part bonding composition such as those of the invention to be most easily used in commercial and industrial environments, the ratio at which the two parts are combined should be a convenient whole number. This facilitates application of the adhesive with conventional, commercially available dispensers. Such dispensers are shown in U.S. Pat. Nos. 4,538,920 and 5,082,147 and are available from ConProTec, Inc. (Salem N.H.) under the tradename xe2x80x9cMIXPACxe2x80x9d and are sometimes described as dual syringe-type applicators.
Typically, these dispensers use a pair of tubular receptacles arranged side-by-side with each tube being intended to receive one of the two parts of the adhesive. Two plungers, one for each tube, are simultaneously advanced (e.g., manually or by a hand-actuated ratcheting mechanism) to evacuate the contents of the tubes into a common, hollow, elongated mixing chamber that may also contain a static mixer to facilitate blending of the two parts. The blended bonding composition is extruded from the mixing chamber onto a substrate. Once the tubes have been emptied, they can be replaced with fresh tubes and the application process continued.
The ratio at which the two parts of the bonding composition are combined is controlled by the diameter of the tubes. (Each plunger is sized to be received within a tube of fixed diameter, and the plungers are advanced into the tubes at the same speed.) A single dispenser is often intended for use with a variety of different two-part bonding compositions and the plungers are sized to deliver the two parts of the bonding composition at a convenient mix ratio. Some common mix ratios are 1:1, 1:2, 1:4 and 1:10.
If the two parts of the bonding composition are combined in an odd mix ratio (e.g. 3.5:100), then the ultimate user would probably manually weigh the two parts of the adhesive. Thus, for best commercial and industrial utility and for ease of use with currently available dispensing equipment, the two parts of the bonding composition should be capable of being combined in a common, whole number mix ratio such as 1:10 or less, more preferably 1:4, 1:3, 1:2 or 1:1.
Bonding compositions of the invention are suited for use with conventional, commercially available dispensing equipment for two-part adhesives. The solubility of the organoborane in the vinyl aromatic compound can be advantageously used to modify the mix ratio of the two parts of the adhesive composition to the most commercially important whole number values (e.g., 1:10, 1:4, 1:3, 1:2 or 1:1).
Once the two parts have been combined, the bonding composition should preferably be used within a period of time less than or equal to the worklife of the bonding composition. The bonding composition is applied to one or both substrates and then the substrates are joined together with pressure to force excess composition out of the bond line. This also has the advantage of displacing bonding composition that has been exposed to air and that may have advanced too far in cure. In general, the bonds should be made shortly after the composition has been applied to the substrate, preferably within a period of time less than or equal to the worklife of the bonding composition. The typical bond line thickness is about 0.1 to 0.3 mm but may exceed 1.0 mm when gap filling is needed. The bonding process can easily be carried out at room temperature and to improve the degree of polymerization it is desirable to keep the temperature below about 40xc2x0 C., preferably below 30xc2x0 C. and most preferably below 25xc2x0 C. Full strength will be reached in about 24 hours under ambient conditions. Post-curing at an elevated temperature may also be used if desired.
The invention will be more fully appreciated with reference to the following nonlimiting examples in which dimensions in English units are nominal and conversion to metric units is approximate.
Various tradenames and abbreviations used in the examples are defined according to the following schedule: